He said the agency had written to the Maharashtra government and the union home ministry to provide more security to officers investigating the shootout case.
Mumbai college girl Ishrat Jahan, Pranesh Gopinath Pilai, Amjad Ali and Jishan Johar were killed by Gujarat Police in a shootout in Ahmedabad in 2004.
CBI sources said that the preliminary chargesheet is likely to name the officers alleged to be directly involved in the incident and the agency will file a supplementary chargesheet on the conspiracy angle in the killing.
The CBI has questioned senior Intelligence Bureau (IB) official Rajendra Kumar in the Ishrat Jahan case, but sources said he is unlikely to be named in the preliminary chargesheet as a conspirator.
Rajendra Kumar was with the IB in Gujarat when the encounter took place.
CBI sources said that the larger conspiracy behind the killings is was still being probed.
The union home ministry has maintained that its prior sanction was required by the CBI to prosecute Kumar. The ministry has also expressed doubts over "evidence" that the agency claims to have of his involvement.
The Bharatiya Janata Party (BJP), the ruling party in Gujarat, contends that the Ishrat Jahan shootout was not fake and was conducted on the basis of information received from IB.
It has also raised questions over the manner in which the CBI was probing the case.